Bitrotting

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Bitrotting

by ErikG92 Feb 2, 2015 8:59AM PST

Ok, first off thanks to those who helped me find more NAS devices, I think I've settled on one, the WD mycloud EX2. This next question is about the phenomenon of "Bitrot". Let me give a little background first. I'm purchasing this device to offer me and my close family a "cloud" for pictures, files, ect. I plan on using two 1 tb hdds in a RAID1 at first, using a usb 3.0 device to back the data up daily. Now onto the questions. This setup is meant to last a long time. The first question is simply, do SSD's defeat the possibility of bit rot? And the second question is as follows. How do i prevent bit rot? My current plan is to replace the hdds yearly. Will this prevent it? Or will the potential corrupt data carry over to the backup, and then just carry over to the new hdds? I haven't decided on the backup device yet.

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Thank you

by ErikG92 Feb 2, 2015 2:36PM PST

Thanks for the info, after looking through your links I've decided to go with two HGST Ultrastar A7K2000's. I missed which exact models they used for the testing, but the numbers for those alone were significant enough to sway me. Now as for SSD's. I found out they are susceptible to data degragation in their own way. Small flaws in insualtion can cause electrical charges to slowly leak, damaging the data. I suppose the real question now is, after multiple years of daily backups and annual disk replacements, can the data ever be so corrupt it's unreadable? Can destroyed bits ever add up to destroy a single picture in my lifetime? Or am I just worrying too much? Keep in mind I want some of these files to be accessible to my great-grandchildren

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(NT) Thank you cont.

by ErikG92 Feb 2, 2015 2:55PM PST

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Sorry

by ErikG92 Feb 2, 2015 2:58PM PST

Sorry pressed the wrong key. I wanted to ask what kind of countermeasures can I use to watch for corrupted data? Say one of the hdds starts failing and starts writing corrupted data, I could potentially not notice and overwrite previous backups with bad ones.

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At this point

by r. proffitt Forum moderator Feb 3, 2015 11:52PM PST

First my background. I was on many teams over the years on surveillance DVR designs so we spent a lot of time on data integrity. I won't be able to cram it all into this space so you may want to go looking for prior works on the subject. For most folk this will never happen. What will happen is the drive will outright fail. Only those that had a single backup in a single place will lose the content. That turns out to be nearly 100% of the losses today.
Bob

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Thank you

by ErikG92 Feb 4, 2015 2:05AM PST

Thank you, that calms my nerves, really. I'm using a RAID1 setup. I just want the files to be preserved for a long time

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r. proffitt · Accepted Answer · Feb 2, 2015 9:09AM PST

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I'm going with no. Here's why.

by r. proffitt Forum moderator Feb 2, 2015 9:09AM PST

CERN did a fine study on errors and you can read it all at http://storagemojo.com/2007/09/19/cerns-data-corruption-research/

They found errors clustering around some areas and they fixed those. HOWEVER my bigger concern is this post:
http://forums.cnet.com/7723-7588_102-634896/hdd-reliabilty-for-2014-is-out-scary/?tag=contentBody;threadListing

So with all that, you are more likely to not only have a drive fail as well as the other errors. At least you got away from that name in that discussion!

SSDs may have other issues since, well, a certain ray can flip a bit in memory or even flash. That's why most systems have a low level CRC or we would get far more complaints.

And yes, errors do carry over to the backups. But most will issue the CRC failure first and the damage would have to be deep as the error correction systems can fix single bit media errors.
Bob

Bitrotting

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